From 0dc32091c4402a85572d792318320a635f6d3836 Mon Sep 17 00:00:00 2001
From: Corey Frang <gnarf37@gmail.com>
Date: Fri, 4 May 2018 13:22:57 -0400
Subject: [PATCH] Delay getting inverse screen CTM until needed

---
 core/constants.js     |  6 ++++++
 core/workspace_svg.js | 22 ++++++++++++++++------
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/core/constants.js b/core/constants.js
index 54ed1fb49..13a90b762 100644
--- a/core/constants.js
+++ b/core/constants.js
@@ -272,3 +272,9 @@ Blockly.RENAME_VARIABLE_ID = 'RENAME_VARIABLE_ID';
  * @const {string}
  */
 Blockly.DELETE_VARIABLE_ID = 'DELETE_VARIABLE_ID';
+
+/**
+ * A constant value that we can use to mark calculated properties as dirty.
+ * @const {string}
+ */
+Blockly.DIRTY = 'dirty';
diff --git a/core/workspace_svg.js b/core/workspace_svg.js
index 84b9fa539..87b4bb900 100644
--- a/core/workspace_svg.js
+++ b/core/workspace_svg.js
@@ -298,24 +298,34 @@ Blockly.WorkspaceSvg.prototype.targetWorkspace = null;
  * @type {SVGMatrix}
  * @private
  */
-Blockly.WorkspaceSvg.prototype.inverseScreenCTM_ = null;
+Blockly.WorkspaceSvg.prototype.inverseScreenCTM_ = Blockly.DIRTY;
 
 /**
  * Getter for the inverted screen CTM.
  * @return {SVGMatrix} The matrix to use in mouseToSvg
  */
 Blockly.WorkspaceSvg.prototype.getInverseScreenCTM = function() {
+
+  // Defer getting the screen CTM until we actually need it, this should
+  // avoid forced reflows from any calls to updateInverseScreenCTM.
+  if (this.inverseScreenCTM_ == Blockly.DIRTY) {
+    var ctm = this.getParentSvg().getScreenCTM();
+    if (ctm) {
+      this.inverseScreenCTM_ = ctm.inverse();
+    } else {
+      // When dirty, and we can't get a CTM, set it to null.
+      this.inverseScreenCTM_ = null;
+    }
+  }
+
   return this.inverseScreenCTM_;
 };
 
 /**
- * Update the inverted screen CTM.
+ * Mark the inverse screen CTM as dirty.
  */
 Blockly.WorkspaceSvg.prototype.updateInverseScreenCTM = function() {
-  var ctm = this.getParentSvg().getScreenCTM();
-  if (ctm) {
-    this.inverseScreenCTM_ = ctm.inverse();
-  }
+  this.inverseScreenCTM_ = Blockly.DIRTY;
 };
 
 /**